Refrigerators, freezers, and similar appliances have evaporator coils upon which moisture condenses and freezes during normal operation of the appliance. These appliances are commonly provided with defrost heaters, which periodically are turned on to heat the coils to a temperature sufficient to melt the accumulated moisture frozen on the coils. The moisture, then as a liquid, drains into an evaporator pan where it evaporates. One common type of defrost heater includes an electrical resistance heating element enclosed within a glass tube which is mounted just below the evaporator coils. Connectors at both ends of the tube connect electrical conductors with the heating element for supplying electrical energy thereto. These heaters are normally mounted just below the evaporator coils. Accordingly, the melting moisture drops onto the heater, must be prevented from reaching the resistance heating element enclosed within the glass tube. However, the resistance heating element in heating the air within the tube causes the air to expand, which will break the tube if not vented. Accordingly, it is necessary to provide a vent for the tube, but this vent also serves as a path through which the melting moisture can reach the inside of the tube and damage the resistance heating element. In fact, some test specifications require that the heater be able to function if entirely immersed in water. Most glass defrost heaters having vents are unable to pass this test, because of water leaking into the glass tube through the vent.